1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus which outputs a color image, for example, by fixing to a transfer material toner images in which single-color images corresponding to color-separated color components are overlaid.
2. Description of the Related Art
In an electrophotographic color image forming apparatus, a certain surface potential is given to a photoconductor capable of holding an electrostatic latent image, the photoconductor surface potential according to a background or image part is changed selectively, a toner image is obtained by supplying a developing agent (toner) to that part, and the toner image is transferred to an output medium (transfer material).
Nowadays, user needs have diversified, and there is a demand to output color images with high quality and exact color reproduction to various media including paper sheets of 50–250 g/m2, transparent film sheets, and adhesive stickers.
A color image forming apparatus includes a black developing unit which outputs a black (Bk) image, and a color developing unit which outputs three single-color images of cyan (C), magenta (M) and yellow (Y) thereby forming a color image.
Toner images comprising four colors formed by respective developing units are sequentially overlaid on a photoconductor or transfer material, that is, a sheet of ordinary paper or an OHP sheet, and fixed to the transfer material by a fixing unit.
A method of increasing the fixing temperature or decreasing the fixing speed when fixing a color image comprising multiple toner images overlaid on a transfer material has been proposed to ensure high-quality color reproduction and a high fixing rate even for thick paper sheets and OHP sheets.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2000-267474 proposes an image forming apparatus which can select a first fixing speed and a second fixing speed higher than the first fixing speed, and decreases the speed of a fixing unit to the second speed by releasing pressure contact between an image holding body and an intermediate transfer body, when fixing a toner image at the second speed to a transfer material whose length is greater than the distance between the intermediate transfer body and the fixing unit, in an apparatus which collectively transfers toner images formed sequentially on an image holding body to a transfer material by transferring them to an intermediate transfer body so that they are overlaid, and then fixes the images to the transfer material by a fixing unit.
In the apparatus disclosed by Jpn. Pat. Appln. KOKAI Publication No. 2000-267474, the pressure contact between the intermediate transfer body and image holding body decreases the speed of the fixing unit.
However, in a method of releasing the pressure contact between the intermediate transfer body and image holding body, overlaid toner images which should be transferred from the image holding body to the intermediate body remain on the image holding body, or the overlaid toner images collapse on the intermediate transfer body, degrading the picture quality. Particularly, in a color image, when part of the toner is insufficient and the toner layer thickness changes, color reproduction becomes inexact.
To decrease the speed of the fixing unit without releasing the contact pressure between the intermediate transfer body and image holding body, it is necessary to make the distance between the fixing unit and the transfer position for transferring a toner image from the intermediate transfer body to a transfer material of a transfer material, greater than the maximum length capable of accommodating an image. This increases the size of an image forming apparatus.
When the fixing speed is increased, it is inevitable to set an upper limit taking into account the maximum value of power applicable to the image forming apparatus in view of the passivity of fire (overheating) if a paper jam should occur in the fixing unit.